118 



PERIDOTITE. 



In the same way the presence of nickel, chromium, tin, copper, and cobalt 

 in the group of terrestrial olivine minerals, serves to connect the earth 

 and meteoric bodies ; as also does the presence of nickel in the terrestrial 

 pyrrhotite and magnetite. 



Section" IV. — The Terrestrial Pcridotites. 



Variety. — Dunite. 



FranJdin, North Carolina. 



5134. An oil-green, crystalline-granular rock, weathering from a yellowish-green to a 

 reddish-brown. Composed of a granular mass of olivine, holding irregular grains and 

 crystals of chromite and long needles of tremolite. It contains some talc and dark-green 

 chlorite. 



Section : a clear, pale-yellowish, fissured mass, composed of olivine, with some talc, 

 chromite, and tremolite. The olivine is in clear transparent grains, tinged slightly yellow 

 aloug the fissures. It contains some chromite and glass inclusions, — the two often being 

 associated. The talc is in clear irregular plates, showing a longitudinal cleavage. The 

 ]iolarization is generally simple, but sometimes aggregate. An earthy, white substance 

 was observed in some cases lying between the laminae. 



The chromite is generally in octahedral crystals, although a few minute grains of irreg- 

 ular form were seen. The chromite was opaque in every instance. A few minute 

 rounded grains were observed, that may possibly be picotite. 



The section, to my mind, presents the characters of a granular rock, resulting from a 

 cooling igneous magma — an eruptive rock. The olivine is in grains which are separated 

 only by fine cracks, every irregularity in one being matched by corresponding irregular- 

 ities in its neighbors. If these grains were olivine sands aggregated by wind or water, 

 such uniformity would not exist. The grains would be irregularly massed togetlier, with 

 interstitial portions filled with binding material. The cracks which separate the different 

 individual grains are the same as those which separate different portions of the same 

 grain. The absence of any signs of wearing to the grains, and their matching one another 

 as they would in this substance when completely crystallized, point towards an eruptive 

 origin for the rock. In addition, long, lenticular, much broken grains are seen, whose parts 

 show in polarized liglit that they belong to the same individual. They are arranged at 

 every angle with one another ; but if these grains had been deposited as a shore sand, it is 

 difficult to see how they could have retained their sharp thin cutting edges. Again, these 

 grains and the general structure of the rock are like those observed in the Estheiwille 

 meteorite, wliich I think no one would be inclined to regard as a beach deposit. The 

 granular structure appears to me to be due to the crystallization of a mineral inclined to 

 take such a rounded form as olivine usually lias. The same structure and arrangement 

 of the grains from a cooling eruptive rock had been previously seen by the writer in quartz 

 in some granitoid rocks from Lake Superior, part of whicli are known to be in dikes, while 

 the others are probably also eruptive.* 



Many of the larger olivine grains show a faint banded polarization, the bands being 

 nearly parallel with a crystallographic axis. The structure of this section is shown in 

 Plate IV. figure 2 ; the darker bands indicating the fissures in the grains. 



* Bull. Mils. Comp. Zool, 18«0, vii. .'):!-r)5. 



